Quantitative analysis of acidic OH groups in zeolite by ammonia IRMS-TPD and DFT: Application to BEA

Abstract

The number and ammonia desorption enthalpy, as a measure of acid strength, of bridging OH groups divided into three portions based on the OH stretching frequency in zeolite β were measured by means of a method of ammonia infrared/mass spectroscopy (IRMS) temperature-programmed desorption (TPD). On the other hand, the optimized structures, wavenumbers and ammonia desorption energies (Edes) of all the geometrically possible acid sites located at crystallographically non-equivalent positions in a BEA structure were calculated with an aid of density functional theory (DFT). The calculated wavenumbers could be classified mainly into three portions. The interaction between H in the AlOHSi unit and oxygen atoms surrounding the unit is suggested to lower the OH frequency. The number, averaged Edes and wavenumber of stretching vibration of each portion were in agreement with the observed values.